1. Field of the Invention
This invention relates to poly(hydroxyacid) compositions and articles manufactured therefrom which have increased rates of polymer depolymerization relative to the conventional polymeric compositions and articles. An aspect of this invention relates to methods for making such compositions. Another aspect of this invention relates to a method for recovering hydroxyacid monomer from the depolymerized poly(hydroxyacid). Still another aspect of this invention relates to a medium, containing the poly(hydroxyacid) composition, from which the monomer can be recovered.
2. Description of Related Art
Poly(hydroxyacids) ("PHA's") such as polyactic acid ("PLA") and polyglycolic acid ("PGA") have been known for many years. Among the important properties of these polymers are their tendency to depolymerize relatively easily and their ability to form environmentally benign byproducts when degraded or depolymerized. Consequently, high molecular weight PHA polymer shaped articles are finding increasing application as replacements for non-degradable polymers such as polystyrene in throw-away products like fast-food containers (Sinclair et al, WO90/01521, published Feb. 22, 1990).
Hydroxy(carboxylic acids) such as lactic acid or their cyclic diesters (in the case of lactic acid the cyclic diester is called "lactide"; in the case of glycolic acid it is called "glycolide") can be polymerized to a PHA such as PLA or PGA. See U.S. Pat. No. 4,797,468 (De Vries), issued Jan. 10, 1989, and also U.S. Pat. No. 4,835,293 (Bhatia), issued May 30, 1989, and U.S. Pat. No. 4,727,163 (Bellis), issued Feb. 23, 1988.
The ease of depolymerization of the poly(hydroxyacids) is generally attributed to ester linkages which are hydrolyzable under mild conditions and/or to greater hydrophilicity as compared to most polyesters. By contrast, typical commercially-used polyesters derived from aromatic dicarboxylic acids and glycols are far less hydrophilic, and their ester linkages are, for all practical purposes, hydrolysis resistant under the type of conditions encountered in a landfill and even under more severe conditions.
The environmentally benign character of the degradation products of PHA's has potential for simplifying or even reducing litter and other waste disposal problems, e.g. in the management of long-term landfills.
While increased use of PHA's can theoretically have a favorable impact on environmental problems, simply discarding high molecular weight PHA articles for destruction by hydrolytic degradation has the cost penalty of discarding the valuable, potentially recoverable hydroxyacid (HA) moieties.
In manufacturing operations in which high molecular weight polymers are shaped to form stock material and particularly to form useful articles, re-use or recycling of factory waste (mold flash, trim waste, etc.) is desirable but not always practical. Theoretically, an ideal way to use polymeric factory waste would be to depolymerize it and recover the monomer or monomers from which it was made.
Although it is clearly impractical (if not impossible) to carry out on-site depolymerization of factory waste made up of only hydrolysis-resistant conventional polyesters, the development of new uses of PHA's suggests a faster route to recovery of valuable monomers from the PHA factory waste, perhaps through the use of an on-site hydrolytic medium, provided the rate of hydrolysis would be rapid enough. Hydrolysis of PHA's is in fact well known, but rates of hydrolysis presently appear to be too slow for treatment of factory waste and the like. In PHA hydrolysis, PHA polymer (including articles formed from PHA) is collected and is typically exposed to water for a long period of time. The end product of the hydrolysis is a low molecular weight oligomer and/or a fundamental acid moiety such as lactic acid (the end product obtained from hydrolysis of PLA) or glycolic (hydroxyacetic) acid (the end product obtained from hydrolysis of PGA). In short, depolymerization of PHA's via the hydrolytic route, though promising, can be very slow.
It has therefore not been possible to achieve depolymerization of PHA's within a time frame which would permit practical recovery and reuse of valuable HA moieties. And even in the case of landfills, the time required for degradation of high molecular weight PHA's is too long to provide a noticeable lessening of the burden on the landfills.
U.S. Pat. No. 5,136,057 (Bhatia), issued Aug. 4, 1992 discloses a process for recovering recyclable lactic acid from impure lactide, and U.S. Pat. No. 5,110,868 (Bellis), issued May 4, 1992 discloses a water-degradable molded object containing low molecular weight (MW.ltoreq.4000) poly(lactic acid/glycolic acid), a nonionic surfactant, and a large content of a water soluble inert salt. Sinclair et al, W090/01521, utilize entrapped lactic acid or lactic acid oligomers to speed up degradation of high-MW PHA's, but more typically the art does not address the problem of providing rapid depolymerization of high-MW PHA (e.g. polymeric articles made from PHA). Nor does the art appear to address adequately the problems of providing practical routes to the recovery of fundamental moieties (e.g. HA's) from high molecular weight PHA's (e.g. from high molecular weight PHA factory waste or from used disposable articles formed from PHA).